An optical fiber is commonly used for communicating data signals over both short and long distances. Compared with other communication mediums, such as a metal wire, an optical fiber is advantageous in that signals travel along it with less loss, and it is also less susceptible to electromagnetic interference. An optical fiber also offers a much higher bandwidth than wire (i.e., data carrying capacity.
One disadvantage of an optical fiber, however, is that it is difficult to couple to another optical fiber or to another component, for example, an electro-optical component, as discontinuities at a connection point may lead to light reflection and losses that degrade signal quality. Another disadvantage for an optical fiber or for a more general form of an optical waveguide is optical loss when the light guiding medium is bent at a relatively sharp angle. Another limitation, which may strongly affect glass optical fiber, is the possibility of breakage or decreased long term reliability when the fiber is held in a bent condition. It is known in the art to use optical fiber to construct an optical backplane for interconnecting board-mounted electro-optical components. Such an application is subject to the limitation discussed herein. For example, a problem in constructing a hybrid electrical and optical circuit board is achieving vertical optical interconnects between optical waveguide layers and optical components or layers on different levels.
This problem is often called the 90-degree bend problem, since light traveling in a waveguide layer that typically includes a planar array of optical waveguides or fibers executes a 90-degree bend to traverse the distance between vertically stacked electrical or optical layers. In cases where the optical layer or layers are built with optical fibers, bending the fiber at 90 degrees may be only accomplished over a relatively large radius, for example, greater than about 4 cm. Alternatively, an abrupt 90-degree bend in the light path can be made by use of a mirror, for example, but unless a focusing optic or another guiding medium is provided at the mirror point, the light will become unguided and will spread out resulting in signal loss as it travels in the vertical direction.
More particularly, optical layers in a circuit board may be fabricated with optical fiber or planar optical waveguides. In the case where an optical layer is fabricated with an optical fiber, an optical fabric may be formed on a flexible substrate, such as a polymer film, so that the fabric can be bent out-of-plane for a vertical interconnection, but with the restriction that the bend radius be large. For planar light guides, a mirror may be included to deflect the light vertically, and external lenses may be employed to focus the light between sources and detectors, thereby accomplishing a 90-degree turn in a reduced space. The fabrication process in this case may involve the assembly of micro-optical elements, which may be relatively costly. Accordingly, when possible, many electro-optical components are edge or side coupled to an optical waveguide.
U.S. Pat. No. 7,218,825 to Jeon et al. discloses an optical waveguide having a curved reflective mirror surface. Partition blocks are beside ends of the optical waveguide. A liquid polymer is dropped between the partition blocks. U.S. Pat. No. 6,611,635 to Yoshimura et al. is directed to an electro-optical interconnect substrate. More particularly, Yoshimura et al. discloses an optical waveguide core on a top surface of a substrate and having an end terminating adjacent to a side surface of an electro-optical device. An upper cladding is formed over the optical waveguide core.
U.S. Pat. No. 5,778,127 to Gilliand et al. discloses an optical transceiver apparatus which has a housing including a diode package aligned with a lens, and having an optical filler composition injected therebetween. The optical filler composition comprises a silicone elastomer which may be used as an index matching element, as a positioning and locking means, or an optical attenuator. Other exemplary approaches for optical fiber connectors or terminations are set forth in the following references: U.S. Pat. No. 5,619,610 to King et al.; U.S. Pat. No. 5,515,465 to Olin et al.; U.S. Pat. No. 6,501,900 to Aloisio, Jr. et al.; U.S. Pat. No. 6,097,873 to Filas et al.; and U.S. Pat. No. 5,058,983 to Corke et al.